1. Field of the Invention
The present invention relates to a multiple-band digital frequency synthesizer receiver. More specifically, the present invention relates to a multiple-band receiver employing a digital frequency synthesizer employing a phase locked loop.
2. Description of the Prior Art
A typical prior art radio receiver comprises as a local oscillator for a tuner a parallel resonance circuit comprising a coil and a capacitor, in which the inductance of the coil or the capacitance of the capacitor is varied to obtain a desired tuning frequency. However, such a local oscillator is liable to suffer from fluctuations of the oscillation frequency caused by temperature characteristics of the coil, capacitor and the other oscillator components. Thus, it is rather difficult to obtain a stabilized oscillation frequency.
A digital frequency synthesizer has also been proposed and in practical use. Such a frequency synthesizer is much more advantageous in that it can provide a much more stabilized oscillation frequency. A typical frequency synthesizer employs a phase locked loop, which is often simply referred to as "PLL".
A frequency synthesizer employing a phase locked loop usually comprises a voltage controlled oscillator the oscillation frequency of which is controllable as a function of an output voltage, as low pass filtered, obtainable from a phase detector, which is adapted to compare the phase or the frequency of the output from a reference oscillator and the phase or the frequency of an output from a programmable frequency divider adapted to frequency divide the output frequency from the said voltage controlled oscillator at the frequency division rate which is adapted to be variable as a function of a control signal. Automatic scanning of the oscillation frequency of the output from the said voltage controlled oscillator is effected by varying the said control signal and thus the frequency division rate of the programmable frequency divider. Therefore, if such a voltage controlled oscillator is used as a local oscillator of a tuner of a radio receiver, automatic scanning of the receiving band can be effected by varying the frequency division rate of the programmable frequency divider, as described above. If and when the frequency of a broadcasting station is tuned by the tuner, an output is obtained by the receiver, which is utilized to disable the variation of the said control signal.
The variable range of the frequency division rate of the programmable frequency divider sould be determined depending on the range of the local oscillation frequency of the local oscillator for a given receiving band, such as an AM medium wave band, AM short wave band, band and, or the like and the frequency difference between the adjacent two broadcasting station frequencies, such that the receiver can receive any broadcasting frequency of the said given receiving band. Thus, it is appreciated that the variable range of the frequency division rate of the programmable frequency divider sould be different depending on the receiving band. For example, the Japanese standard for FM broadcasting has been determined as a range of receiving frequencies from 76 through 90 MHz, an intermediate frequency of 10.7 MHz, and an inter-station frequency of 100 KHz. This means that the local oscillation frequency should be varied from 65.3 through 79.3 MHz, assuming that a lower sideband superheterodyne is employed. Accordingly, it is necessary that the frequency division rate of the programmable frequency divider be variable from 653 to 793. On the other hand, the Japanese standard for AM medium wave broadcasting has been determined as the range of receiving frequencies from 535 through 1605 KHz, an intermediate frequency of 455 KHz and an inter-station frequency of 10 KHz. This means the local oscillation frequency should be varied from 990 to 2060 KHz and accordingly the frequency division rate of the programmable frequency divider need be varied from 99 to 206, assuming that an upper sideband superheterodyne is employed.
Assuming that the receiving band is switched from a state of receiving FM broadcasting to a state of receiving AM broadcasting, the difference between the variable range of the frequency division rate of the programmable frequency divider for reception of FM broadcasting and the variable range of the frequency division rate of the programmable frequency divider for reception of AM broadcasting makes impossible the reception of AM broadcasting because of the receiving frequency in such a situation being outside of the range of the receiving band of AM broadcasting, if and when a receiver designed for reception for FM broadcasting is simply utilized for reception of AM braodcasting, and vice versa. Thus, in implementing a multiple-band radio receiver, it is necessary to provide a multiple number of frequency synthesizers each of which is adapted for reception of the corresponding broadcasting band. However, provision of a multiple number of frequency synthesizers makes the structure of the receiver complicated. It is desired that a multiple-band receiver with simplified structure be provided. Further it is desired that a multiple-band receiver is provided in which automatic scanning of the receiving band is effected from either the minimum or the maximum frequency of the receiving band when the receiving band is switched.